TRICHONISCIDAE - Brackenridgia nr. sphixensis
Brackenridgia nr. sphinxensis
Size: 3.74 mm
Group Guild Status
Troglobiont Scavenger Common
This is one of the two truly troglobiotic species that was recognized as such in the initial study. We do not know whether the Kartchner population represents a new species, or is a disjunct population of B. sphinxensis, which was originally described from “Sphinx” (ne - Spinks) Cave in the Chiricahua Mountains (Schultz 1984). Apparently the taxonomic work that exists for this genus is in disarray, and a revision of the group is needed. This will be a major undertaking and even once the work is begun, it may take years before there is some closure on the taxonomy of Brackenridgia. Only then will we know where the Kartchner population fits into the regional taxonomy of the group.
Brackenridgia (Photo 1) are generalist scavengers and occur in most areas of the cave wherever organic nutrient resources are available within areas of suitable habitat. The animals seem to be present only at sites where obvious fungi and molds have developed, such as on bat guano deposits, on decaying wood (Photo 2), and on dead animals including bats and invertebrates. They have not been found where these resources do not occur, and it is not known whether the animals can subsist solely on microbial nutrients. During the recent two-year study they were never observed on the two primary bat guano deposits, and may avoid these areas due to threat of predation, a high level of competition for resources, or other factors. Even when the invertebrate activity at the major guano deposits becomes senescent (from September to March), and the threat of predation may be reduced, they do not occupy these areas. The animals appear tolerant of saturated surface conditions, and do not occur in drier areas of the cave such as the vicinity of the Entrance Sink.
A review of our two years of data for this species has shown that there is an increase in the animals’ activity in response to the influx of autogenic meteoric waters from both periods of our regional bimodal precipitation regime. Based on 65 sightings of the animals during all months of the year the population peaked in April subsequent to the winter/spring precipitation season and again from September through November after the summer rains. There appears to be about a two week lag in the response time between the peak of the summer rains and the upward trend of the Brackenridgia population. The delay likely represents the time required for mold to develop to a level where it becomes attractive to the animals. Since this is such a short time period, it seems likely that the animals were dispersed or sequestered nearby in the cave, and reappear when conditions become favorable, congregating at the food sources when they begin decomposing. The species is locally and seasonally common in the cave.
The initial study stated that the distribution of the animals in the cave was “patchy”, but that they were the most widely dispersed species within the cave, occurring “….in nearly every part of the cave except the Throne Room and the upper Portion of the Rotunda Room.” Frass (fecal pellets) from these animals was found at the sites of old wood trail markers (popsicle sticks) that had been placed by the original explorers (Photo 3). The study also mentions that this was the only species regularly encountered in the Back Section of the cave. Interestingly, during the recent study we found these isopods in the Back Section of the cave on only four occasions, in the lower Rotunda Room on July 17, 2010, in Granite Dells on February 12, 2011, and in the Pyramid Room on February 12 and July 30, 2011. All of these observations except the one in the lower Rotunda Room were at the wood blocks at the invertebrate stations in those areas. The Rotunda Room observation was at lampenflora on the clay floor near the tour lights at the beginning of the "discovery trail" (beyond the lake in the Rotunda Room).
During the initial study the animals were apparently found feeding on organic materials (“….twigs, leaves, and other plant material”) washed into the cave during major surface flow events in Guindani and Saddle washes. During the recent study we saw no evidence of in-wash debris in these areas, even considering that there was a major surface flow event in the winter of 2009/2010 that flooded the Back Section of the cave, which was equal to one that occurred during the initial study (Photo 4). We suspect that the influx hydrology of waters entering the back portions of the cave may have changed so that coarse organic debris is no longer an allochthonous nutrient input to the cave. This could have resulted from a structural collapse at the cave perimeter, or simply filling of voids by sedimentation, which could block larger materials from being transported into the cave. This may be a temporary condition or a relatively permanent change, and appears to have affected the abundance of Brackenridgia in the Back Section of the cave. Placement of our wood block baits has shown the species is still present in the Back Section, but apparently in reduced numbers. Alternatively, if coarser organic materials are unavailable, the animals may be surviving on microbial nutrients and only “appear” at any food sources that become available. In this sense, the historic in-washed organic debris would be analogous to our recently placed wood blocks. In either case the animals may be observed only because we have provided a food source to which they are attracted.
Juveniles are present year round and reproduction is probably either opportunistic, associated with adequate available food sources, or possibly consisting of two overlapping generations associated with the two peaks of activity coupled with the regional precipitation regime.
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